In the field of optical communications, the use of the Mach-Zender optical modulator is well-known. The Mach-Zender optical modulator mixes an RF information-bearing signal with a lightwave carrier by electromagnetic phase interferometry. Upon entering the modulator, the lightwave carrier is typically split into two signals that are coupled into separate waveguides formed in the crystal structure of the modulator. Electrodes are placed in close proximity to the waveguides in the device. An RF information-bearing signal is applied to the electrodes next to one of the waveguides. The propagation of the lightwave carrier through the crystal is affected by electric field variations that the RF signal causes in the propagation characteristic of the waveguide in the area near the electrodes. The electric field causes a local change in the refractive indices around the waveguides, thereby speeding up the propagation of the wave in one path while delaying the other. Thus, the relative phase of the two lightwave signals in the modulator is changed in proportion to the modulating signal applied to the electrodes.
At the output of the modulator the divided carrier signals are recombined. When the two signals having variations in relative phase caused by the RF input are recombined, phase interference occurs. Some of the interference is destructive and some constructive. This produces a modulated lightwave output having amplitude changes in proportion to the modulating RF signal. The modulated carrier can be coupled to a fiber optic medium for transmission over considerable distances.
An optical modulator, like its semiconductor counterparts in RF electronics, is a non-linear device. The typical Mach-Zender optical modulator comprises a lithium niobate (LiNbO.sub.3) crystal device having a non-linear modulation characteristic. In order to optimize the quality of the modulated output from an electro-optical modulator, it is desirable to apply a bias control to the device to set its operating point, or bias point, as close as possible to the center of its linear range.
The deviation of the modulator transfer function from the linear response of the ideal modulator causes odd-order harmonic distortion. Because the principle of operation of the Mach-Zender modulator is phase interferometry, the center bias point is very sensitive to temperature, input signal fluctuations, and manufacturing tolerances. If not properly biased, the modulator will generate even-order harmonics and increased odd-order harmonics. Therefore, for the best performance, it is necessary to continuously monitor the output of the modulator and update the bias to ensure the least harmonic distortion and, thus, the maximum dynamic range of the optical communications link. A feedback control providing the bias signal accomplishes this task.